1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to polishing heads for use in the chemical-mechanical polishing of semiconductor wafers.
2. Background of the Related Art
The manufacture of an integrated circuit device requires the formation of various layers (both conductive and non-conductive) above a base substrate to form the necessary components and interconnects. During the manufacturing process, removal of a certain layer or portions of a layer must be achieved in order to pattern and form various components and interconnects. Chemical mechanical polishing (CMP) is being extensively pursued to planarize a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. It is also used in flattening optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to polish the exposed surface of the wafer (or a layer formed on the wafer), when subjected to a force pressing the wafer onto the pad. In the most common method of performing CMP, a substrate is mounted on a polishing head which rotates against a polishing pad placed on a rotating table (see, for example, U.S. Pat. No. 5,329,732). The mechanical force for polishing is derived from the rotating table speed and the downward force on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery as well in averaging the polishing rates across the substrate surface.
Another technique for performing CMP to obtain a more uniform polishing rate is the use of a linear polisher. Instead of a rotating platen and pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the global planarity is improved over CMP tools using rotating pads. An example of a linear polisher is the TERES polisher available from Lam Research Corporation of Fremont, Calif.
Unlike the hardened table top of a rotating polisher, linear polishers are capable of using flexible belts, upon which the pad is disposed. This flexibility allows the belt to flex, which can cause a change in the pad pressure being exerted on the wafer. When this flexibility can be controlled, it provides a mechanism for controlling the polishing rate and/or the profile. Accordingly, a fluid platen can be readily utilized to control the pad pressure being exerted on a wafer at various locations along the wafer surface. Examples of fluid platens are found in the aforementioned TERES polisher and in U.S. Pat. No. 5,558,568.
With either type of polisher (linear or rotary), the polishing head is an important component of the polishing tool. The polishing head provides means for holding and supporting the wafer, rotating the wafer and transmitting the polishing force to engage the wafer against the pad. Generally, the polishing head includes a housing in which a wafer carrier resides. The wafer carrier and/or the head housing is coupled to a rotating mechanism so that the wafer can rotate.
The wafer is mounted on the carrier and held in place by a retainer element, such as a wafer retaining ring. A thin seating material (insert) may be utilized on the mounting surface of the carrier to cushion the seating of the wafer. When in operation, the carrier may have one or more height positions. For example, one height position relative to the housing can be for the mounting of the wafer onto the carrier assembly, while a second height position of the carrier is used when the wafer is to engage the polishing pad.
Generally, when the wafer is being polished, the downforce exerted by the polishing head assembly should be of sufficient magnitude to press the wafer onto the pad so that CMP can be performed. When linear polishers are utilized, they generally employ a flexible belt/pad assembly, so that a fluid platen can exploit this flexible property. The fluid flow from the fluid platen can compensate (or adjust) the pressure exerted by the polishing pad in engaging the wafer.
Likewise, this flexibility can be incorporated in a polishing head as well. By using a flexible diaphragm (or membrane) to couple the carrier to the head housing, the wafer carrier can be made to flex. One such polishing head utilizing a flexible diaphragm, although for a rotating table polisher, is disclosed in U.S. Pat. No. 5,205,082. The use of a flexible membrane allows the carrier to flex in the vertical direction. By ensuring a steady positive pressure on the carrier, a steady downforce can be maintained to engage the wafer on the pad.
A problem with using the known flexible membrane polishing head is that the flexibility, which improves the distribution of the downforce in the vertical direction, also introduces undesirable movement in other directions. Since there typically is a gap between the floating carrier and the sidewalls of the head housing assembly, there is a tendency for the carrier to move in the horizontal direction. This is especially pronounced with a linearly moving belt of a linear polisher. Since the belt/pad assembly always moves in the same horizontal direction, the horizontal force exerted on the wafer causes the carrier to move horizontally as well. Sometimes, the carrier will also pivot about an axis perpendicular to the horizontal axis, due to the horizontal force of the moving belt. The horizontal and/or the pivoting movement of the carrier can introduce uncertainties in the polishing performance, which can impact on the polishing uniformity. The polishing uniformity may not be compensated strictly from the averaging obtained by rotating the wafer. Thus, it is desirable to inhibit or reduce the flexible movement of the floating carrier in the horizontal direction, so that the center of the carrier will reside at the center of the head assembly during polishing.
The present invention describes a polishing head in which a floating (flexible) carrier is used, but one in which the horizontal travel of the carrier is restricted.
According to one aspect of the present invention, a polishing head for performing chemical-mechanical polishing on a linear polisher is described, in which a flexible diaphragm coupling is used between a wafer carrier and a housing unit housing the wafer carrier. The flexible diaphragm is disposed across the carrier and the housing to permit the carrier and the wafer mounted thereon to move vertically, but restricts the carrier so that horizontal travel in the direction of a linearly moving belt is limited. A retainer ring for retaining the wafer on the carrier is affixed to the carrier itself so that it will not move independently from the carrier, in order to limit wafer slippage. The carrier is subjected to pressurized air or gas in order to maintain engagement of the wafer onto the pad.
In one embodiment, the carrier assembly is a singular unit but in an alternative embodiment, the carrier assembly incorporates a removable subcarrier. However, in both instances, the flexible diaphragm allows the wafer to move vertically, but the center shaft restricts horizontal movement of the wafer during polishing.